Outgoing Mail Server says "offline"

My outbound emails have not been going out (I am on a Mac desktop and use AT&T for email).  Not sure why this happened-just started happening.  I know to look under Mail, then Preferences, Accounts, Server Setting and I am looking at att.net (imap).  Here is says Outgoing Mail Account Offline.  I click on drop down and select Edit SMTP Server List and a large drop down opens up with these fields and nothing is filled in and it won't let me fill anything in.  I also don't know what to fill in here anyway.  Under "Server Settings" these are all blank:  Description, User Name, Password, Host Name, Port, Authentication.  Is this the problem? Also under "Advanced" is says TLS Certificate and "none" is filled in. I also can't alter this.  Doesn't let me if I try. Would love input if this is why my outbound email has not been working?

One other caveat- i have been repeatedly getting these "pop ups" from Yahoo asking me to update my password and I have been ignoring it bz I don't use Yahoo email and dislike it very much.  But as soon as I put it my password, all my emails in my "outbound box" went through. But, as soon as I drafted a new email and sent it, a drop down came up and said I don't have an email server selected (this part I don't recall exactly) but I had to choose att and then it went through.  I went back to Server Settings to check and see if all those fields had been filled in, since I update my password and no, they are still blank and it still says my outbound is offline!  Help please! 🙂